In general, hydrocarbon-bearing zones are difficult to detect in laminated clastic reservoirs consisting of alternating thin layers of shale and sand, when using a conventional induction logging tool. If the layers are too thin to be detected individually by the induction logging tool because the vertical resolution of the tool is too low, the tool reads an average conductivity of the formation. In a vertical borehole, a conventional induction logging tool with the magnetic dipole moments in vertical direction reads an average conductivity which is a combination of the sand- and shale conductivities, but which is dominated by the relatively high conductivity of shale layers. A transversal induction logging tool with the magnetic dipole moments in horizontal direction reads an average conductivity which is dominated by the relatively low conductivity of the hydrocarbon-bearing sand layers.
If the volume of shale/sand in the formation is known, e.g. from measurements using a gamma-ray tool or a nuclear tool, a combination of a conventional induction logging tool and transversal induction logging tool can be used to determine the conductivities of the individual shale- and sand-layers, provided the layers of the same type have the same conductivity. However, the response of the transversal induction logging tool is difficult to interpret because its response suffers from large borehole-effects and wellbore fluid invasion-effects.
A known method of reducing the borehole-effects from the response of transversal induction logging tool is disclosed in Tabarovskii et al, 1979, "Radial characteristics of induction focusing probes with transverse detectors in an anisotropic medium", Soviet Geology and Geophysics, 20, pp. 81-90. In this method an induction logging tool is used, which tool comprises magnetic field transmitter means suitable to induce a magnetic field in the formation, and magnetic field receiver means suitable to receive response magnetic fields at different spacings from the transmitter means and to provide a signal representative of each response magnetic field. However, the logs obtained when using this tool are rather "wild" and are therefore difficult to interpret. This problem becomes even more manifest when the tool is run through a sequence of earth layers.
It is therefore an object of the invention to provide a method of determining a characteristic of an earth layer of a laminated earth formation, in which a log response is obtained which is easy to interpret and which provides an accurate indication of the characteristic even if the thickness of the individual earth layers is relatively thin. Furthermore, it is desirable that the relative orientation of the logging tool with respect to the formation layering is determined.
It is a further object of the invention to provide a system for carrying out the method according to the invention.
It is another object of the invention to provide an induction logging tool for use in the method and system according to the invention.